Thousands of people have been breathing in carcinogenic toxins from the smouldering ruins of the World Trade Centre, according to internal US government reports.

On one day alone, the level of benzene, which can cause leukaemia and bone marrow damage with prolonged exposure, was measured at 58 times official safety levels.

On a another day, monitors found the chemical at 42, 31 and 16 times the Occupational Safety and Health Administration's limits at three spots around the site in lower Manhattan.

Other toxic substances detected at unsafe levels in the air and soil around the site, which is still burning more than six weeks after the attacks, include sulphur dioxide, dioxins, PCBs and lead, say the documents obtained by the New York Daily News.

"Yes, they are high," said an Environmental Protection Agency spokeswoman. "But you get a little distance from the plume and they go dramatically down."

Sulphur dioxide was found in the air at above the safety level on numerous occasions; one day last month lead was present at three times the limit; on another, discharges of dioxins into the Hudson river were five times higher than any recorded in New York harbour.

Concern had focused on asbestos, which was used in the lower of the towers' 110 storeys. Studies reveal that the amounts in the air are within agreed safety standards but there are fears that some fibres are so small that they may evade detection.

"What I've seen of the data is troubling," said Paul Bartlett, an expert on dioxins and PCBs at Queens College centre for the biology of natural systems in New York.

"They aren't treating this as a disaster so they're not asking to what extent and how far are people being exposed or who is possibly being affected by the release of chemicals. They're just checking what emissions are exceeding regulations."

Victims' families will be beside the site tomorrow to attend a memorial service.

Another report found that immediately after the catastrophe, few rescue workers wore respirators, although most do now.

The National Institute of Environmental Health Sciences says almost 1,000 injuries at Ground Zero, from blisters to burns and fractures, might have been prevented if more rescue teams had received basic training and more had worn hardhats.

"There is no excuse for what I saw," said John Moran, the consultant who drew up the report.

"When I was up there there was no evidence of any safety or health programme or plan. It's the worst site I've ever seen, extremely hazardous. Very few of the workers were wearing even the most basic protective equipment."

But Joel Shufro, executive director of the New York Committee on Occupational Safety and Health, said the firefighters, police officers, rescue and construction workers were dealing with an unprecedented event.

"They were taking risks which, under the circumstances, were understandable and those people need to be considered heroes," he said. "What needs to happen now is that workers need to be protected so they don't suffer illness or injuries.